Do we know what kind of impact this will have on the web at large? Fixing abusted video on our own landing page is one thing. But if the majority of videos on the web are similarly affected, we have a problem.

We've had this change in for around a month, and as far as I know this is the only bug that has been filed on (related) video problems.
That said, I have a better patch for bug 631058 mostly complete (need to figure out why it is failing a few tests), and would be fine reverting this change in Fennec.Next.